By the way, I don't have the impression that packagers are expected to
make sure that upgrades like directly from FC(x) to FC(x+2) without
stopping at FC(x+1) do work in the first place. Other opinions?

We definitely need to make sure that works. Many uses skip releases
because they dont want to upgrade so soon and they didnt find any
benefits in the latest release or there are some regressions and things
like that. Upgrade paths to newer releases should always be as smooth as
possible (not just between x and x+2).